1. Field of the Invention
The present invention relates to a sheet transport apparatus and an image forming apparatus, and more particularly to a sheet transport apparatus in an image forming apparatus having registration means which feeds sheets to image forming means.
2. Description of the Relates Art
A conventionally known image forming apparatus such as copying machines, printers, facsimile machines and a multifunction machines thereof includes an image forming unit for forming an image on a sheet, a sheet transport unit for feeding a sheet to the image forming unit, and a sheet discharging unit for discharging the sheet on which the image was formed by the image forming unit out from the image forming apparatus. There is also a known image forming apparatus having a sheet inverting transfer path which inverts a sheet on which an image was formed by the image forming unit to form images on both surfaces of the sheet and which feed the sheet to the image forming unit again.
A conventional example of such an image forming apparatus will be explained with reference to FIGS. 14 and 15. FIG. 14 is a schematic sectional view of an electrophotographic type copying machine as a conventional image forming apparatus. This electrophotographic type copying machine (simply copying machine, hereinafter) includes a document reading unit 4 for reading an image of an original, an image forming unit for forming an image on a sheet in an electrophotographic type based on image information read by the document reading unit 4, and a plurality of sheet transport units, and the copying machine can feed a plurality of sheets from each sheet transport unit to the image forming unit continuously.
In FIG. 14, a reference number 1 represents a copying machine body. An original base plate 2 comprising a clear glass plate is fixed to an upper portion of the copying machine body 1. A reference number 3 represents an original supplying apparatus. The original supplying apparatus 3 conveys originals to a predetermined position of the original base plate 2.
In the copying machine, an image of the original on the original base plate 2 is read by the document reading unit 4 and based on this data, a writing laser unit 5 in the image forming unit radiates laser light and scans the uniformly charged photosensitive member drum 12, thereby forming an electrostatic latent image on the photosensitive member drum 12.
The copying machine is provided with deck supply units 34 and 35 and cassette supply units 36 and 37 which supply decks 30 and 31 incorporated in the copying machine body 1 and sheets loaded on sheet cassettes 32 and 33.
The image forming unit includes the photosensitive member drum 12, a charger 13 for uniformly charging a surface of the photosensitive member drum 12, a development unit 14 which develops an electrostatic latent image formed on the surface of the photosensitive member drum 12 which is charged by the charger 13, and which forms a toner image to be transferred to a sheet S, a transfer charger 19 for transferring the toner image developed on the surface of the photosensitive member drum 12 to the sheet S, a separation charger 20 for separating the sheet S on which the toner image is transferred from the photosensitive member drum 12, and a cleaner 26 for removing toner remaining on the photosensitive member drum 12 after the toner image is transferred.
The image forming unit is provided at its downstream side with a transport unit 21 for transporting the sheet S on which the toner image is transferred, and a fixing unit 22 which fixes the toner image on the sheet S transported by the transport unit 21 as a permanent image. The image forming unit further includes an output roller 24 for discharging the sheet S on which the toner image is fixed by the fixing unit 22 from the copying machine body 1. The copying machine body 1 is provided at its outer side with an output tray 25 for receiving the sheet S discharged by the output roller 24.
In the copying machine, transport rollers 105, 107, 108, 109, 110, 111, 112 and 123 as sheet supply units, and sheet path sensors 104, 116, 117 and 118 for detecting a leading end and a rear end of the sheet are disposed on a transport path on which the sheet is transported from the sheet transport unit to the image forming unit. The sheet supplied by the transport rollers is fed to the image forming unit by a registration roller 106. A registration sensor 120 for detecting the leading end of the sheet is disposed in the vicinity of the upstream side of the registration roller 106 in the sheet conveying direction.
Here, the transport roller 107 is a pre-registration roller for feeding a sheet transported from the sheet transport unit to the registration roller 106. The first transport roller 105, the sheet path sensor 104, the second transport roller 108, the third transport roller 109, the sheet path sensor 116, the fourth transport roller 110 and the sheet path sensor 117 are disposed in this order on a sheet transport path upstream side of the pre-registration roller 107.
A sheet transport path connected to the deck supply unit 35 is branched from the transport path between the pre-registration roller 107 and the first transport roller 105. The fifth transport roller 111, the sheet path sensor 104, a sixth transport roller 112, a sheet path sensor 118 and a seventh transport roller 123 are disposed in this order on this sheet transport path.
A sheet inverting path is branched off from a transport path between the sixth transport roller 112 and the seventh transport roller 123. A sheet path sensor 119, a both-sided right roller 113, a both-sided left roller 114, an inverting roller 115 which rotates in both normal and reverse directions, and a sheet inverting unit 121 are disposed in this order from downstream side of the sheet inverting path. In this copying machine, to form images on both surfaces, a sheet on which an image is formed by the image forming unit is inverted by the sheet inverting unit 121 and the inverting roller 115 and fed to the sheet inverting path, and is again supplied to the image forming unit through the both-sided left roller 114, the both-sided right roller 113, the sixth transport roller 112, the fifth transport roller 111, the pre-registration roller 107 and the registration roller 106 in this order.
A driving force of a driving motor (not shown) is transmitted to the above-described rollers so that the rollers are rotated and driven. The rotations of the rollers are controlled by a control unit (not shown) based on detection results of the sheet path sensors.
Next, the operation of the conventional copying machine when a sheet is supplied will be explained with reference to FIG. 15 based on a case in which the sheet is supplied from a deck supply unit 34. FIG. 15 is a diagram showing a positional relation and the like of a leading end and a rear end of a sheet when the sheet is supplied from the deck supply unit 34.
When the supply of sheets is started, a pickup roller 101, a supply roller 102, a separation roller 103, the first transport roller 105 and the pre-registration roller 107 are rotated and driven by the driving motor. At that time, the registration roller 106 is still in its stopped state.
When the sheets are supplied, sheets S set in the sheet deck 30 are supplied to the supply roller 102 by the pickup roller 101. The sheet S is provided such as to be opposed to the supply roller 102, and a force rotating in the opposite direction to the conveying direction is given to the separation roller 103 with constant torque. Only the uppermost sheet is separated from the sheets S by the separation roller 103.
The leading end of this uppermost sheet is detected by the sheet pass sensor 104, and is conveyed by the first transport roller 105.
At that time, to adjust a interval between sheets, i.e., to carry out a so-called pre-registration operation, control is performed in such a manner that the rotation of the first transport roller 105 is temporary stopped based on the detection timing of the sheet leading end by the sheet pass sensor 104, the leading end of the sheet which is being transported by the first transport roller 105 is stopped at a predetermined position on the transport path, and after a predetermined time is elapsed, the rotation of the first transport roller 105 is restarted.
That is, in the sheet transport unit, a sheet whose leading end is in a normal loading position of the sheet deck 30, and a sheet whose leading end is located near the supply roller 102 are also supplied, the leading end positions when the supply is started are varied, and it is necessary to eliminate this variation at the upstream side of the registration roller 106.
More specifically, as shown in FIGS. 14 and 15, the first transport roller 105 is temporary stopped with timing at which the sheet leading end comes on the predetermined position (pre-registration position) A on the transport path after the sheet leading end is detected by the sheet pass sensor 104, and the stop position of the sheet leading end is confirmed. Then, control is performed to restart the first transport roller 105 based on time B (B=(distance C to the registration roller 106)/(sheet conveying speed β of the first transport roller 105 required until the sheet reaches the registration roller 106) at which it is assumed that the sheet leading end reaches the registration roller 106.
Thereafter, the sheet reaches the registration roller 106 through the pre-registration roller 107. Here, the sheet is supplied by a constant amount by the pre-registration roller 107 in a state which the leading end of the sheet butts against the stopped registration roller 106, and the entire sheet forms a loop, thereby correcting the skew of the sheet. Then, the registration roller 106 starts rotating at a constant speed (process speed) α, the sheet is fed to the image forming unit with timing of the image forming operation, and a toner image is transferred on the upper surface.
The sheet which passed the image forming unit is fed to the fixing unit 22 by the transport unit 21 and a toner image is fixed thereon. In the case of a single-sided copy, the sheet is placed on the output tray 25 through the output roller 24. In the case of a both-sided copy, the sheet is inverted by the sheet inverting unit 121 and then, the sheet is conveyed on the sheet inverting path by the inverting roller 115, the both-sided left roller 114 and the both-sided right roller 113, and is fed to the image forming unit again.
Here, as timing at which supply of second and subsequent sheets is started at the time of continuous feeding, the supply is started after a constant time is elapsed after the immediately preceding sheet restarted from the first transport roller 105, and the supply is controlled in the same manner as described above.
When sheets are continuously supplied from the other deck supply unit 35 or the cassette supply units 36 and 37 also, the same pre-registration operation is carried out. When sheets are transported from any one of the sheet transport unit and the both-sided copy is carried out, the rotation of the both-sided right roller 113 is temporary stopped between the sheet path sensor 119 and a merging portion between the deck supply unit 35 and the both-sided transport path based on detection timing of the sheet leading end by the sheet path sensor 119 on the both-sided transport path, and the pre-registration operation for restarting the rotation after a constant time is elapsed is carried out.
In this manner, according to the conventional copying machine, the variation in the sheet leading end position at the sheet transport unit is eliminated by carrying out the pre-registration operation, the position of the sheet leading end is defined, and the sheet is fed to the registration roller 106 stably. The conveying speed β on the transport path from the sheet transport unit to the registration roller 106 is set faster than the sheet conveying speed (process speed α of the registration roller 106) at the image forming unit so that even when the sheet is temporary stopped in the transport path by the pre-registration operation, the next sheet can catch up the immediately preceding sheet.
Japanese Patent Application Laid-open No. 2002-029649 discloses the following control.
In a high speed copying machine, the need for writing an image within a previously printed frame is increased, and deviation (0.5 mm or less) of a position where an image with respect to the sheet is written becomes a problem. Such positional deviations include three elements, i.e., a “leading end registration deviation” which is a positional deviation of the leading end in the sheet conveying direction, a “lateral end registration deviation” which is a positional deviation of an end of the sheet in the widthwise direction perpendicular to the sheet conveying direction, and an “skew feeding” in which a sheet is inclined in the sheet conveying direction. In the conventional example, the “leading end registration deviation” is caused by a difference in an entering degree of a sheet into the roller nip portion when the sheet butts against the registration roller (a thick sheet does not enter to the depth portion of the nip portion of the registration roller, and a thin sheet enters the depth portion of the nip portion), and is varied depending upon a connection variation of an electromagnetic clutch which starts rotation of the registration roller. The “lateral registration deviation” is varied due to positional deviations of the decks 30 and 31 and the each sheet cassettes 32 and 33, and due to insufficient pressing force of a pressing member (not shown) which presses the sheet in these units. The “skew feeding” has a problem that if the sheet butts against the registration roller to form a loop to correct the skew of the sheet, the sheet can not be corrected skew when the sheet has large skew amount.
To solve the problems of the conventional technique, it is conceived that skew feeding correcting apparatuses 200a, 200b, 201a and 201b and image writing position shifting apparatuses 202 and 203 as shown in FIGS. 16 and 17 are provided. The skew feeding correcting apparatuses correct the skew feeding of the sheet S utilizing a speed difference between the skew correcting rollers 200a and 200b whose speeds can independently be controlled, and the skew feeding correcting apparatuses obtain the speed difference from information of the two correction sensors 201a and 201b provided near the downstream sides of the rollers 200a and 200b and perform the control. Each of the image writing position shifting apparatuses comprises a leading end detection sensor 202 which detects the leading end of the sheet in the conveying direction, and a CIS sensor 203 which detects a position of an end (lateral end) of the sheet in the widthwise direction perpendicular to the conveying direction, and the image writing position shifting apparatus is disposed such that it can detect the position of the sheet before the timing at which the laser unit 5 starts writing on the photosensitive member drum 12. That is, the image writing position shifting apparatus is disposed downstream side from the skew feeding correcting apparatus and upstream side from the pre-transfer driving roller 309 which feeds the sheet, and the image writing position shifting apparatus moves the writing position of the laser unit 5 with respect to the photosensitive member drum 12 by a control unit (not shown) based on the information of these two sensors 202 and 203. With this, it is possible to realize the writing position with high precision, and to write an image on a sheet within a previously printed frame with high precision.
However, even if the interval between the sheets is adjusted by the pre-registration operation, transfer variation after the pre-registration operation generates slight deviation of interval between sheets. In the case of a static skew feeding correction in which skew feeding is corrected while butting a sheet leading end against the registration roller and stopping the sheet leading end after the pre-registration operation as shown in FIGS. 14 and 15, if the rotation starting time of the temporarily stopped registration roller is adjusted, it is possible to adjust the deviation of interval between sheets, but in the case of dynamic skew feeding correction in which the skew of the sheet is corrected by the speed difference between the two skew correcting rollers without stopping the sheet leading end as shown in FIGS. 16 and 17, especially when the number of sheets (PPM (Page Per Minute) hereinafter) to be printed per one minutes is high like a high speed copying machine, PPM is not stabilized by the slight deviation of interval between sheets, and even if the high precision writing position with respect to the sheet can be realized, there is a possibility that the productivity is not stabilized.